BUSINESS SEGMENT INFORMATION Business Segment Information (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Segment Reporting Information [Line Items]
Percent of Revenue Contributed	100.00%	100.00%	100.00%	100.00%
Intercompany revenue elimination	$ (2.2)	$ (0.4)	$ (4.5)	$ (0.9)
Revenues	2,831.3	2,621.4	8,545.9	7,563.3
Operating Income (Loss)	343.4	326.7	1,018.0	974.6
Earnings before income taxes	499.6	266.8	1,053.8	811.3
Provision for income taxes	499.6	266.8	1,053.8	811.3
Provision for income taxes	180.6	92.5	328.1	268.6
Net earnings	319.0	174.3	725.7	542.7
Net income attributable to Laboratory Corporation of America Holdings	(0.2)	(2.8)	0.1	(3.4)
Nonoperating Income (Expense)	156.2	(59.9)	35.8	(163.3)
Net Income (Loss) Attributable to Parent	318.8	171.5	725.8	539.3
Corporate Segment [Member]
Segment Reporting Information [Line Items]
Operating Income (Loss)	$ (33.9)	$ (37.9)	$ (106.5)	$ (105.2)
LabCorp Diagnostics [Member]
Segment Reporting Information [Line Items]
Percent of Revenue Contributed	62.00%	68.00%	62.00%	67.00%
Revenues	$ 1,752.0	$ 1,754.7	$ 5,336.3	$ 5,115.5
Operating Income (Loss)	$ 289.4	336.6	$ 929.2	982.0
Covance Drug Development [Member]
Segment Reporting Information [Line Items]
Percent of Revenue Contributed	38.00%		38.00%
Revenues	$ 1,081.5	867.1	$ 3,214.1	2,448.7
Operating Income (Loss)	$ 87.9	$ 28.0	$ 195.3	$ 97.8